Inoculating a horse against a disease causes his immune system to mount a response that on certain blood tests can look like natural infection. So, this begs the question: When dealing with a potentially deadly mosquito-borne disease, such as Eastern equine encephalomyelitis (EEE) or West Nile virus (WNV), how do you tell the difference between a horse that’s been vaccinated and one that’s truly infected?

“This is a common question to the technical services veterinarians at the vaccine companies and to veterinarians,” explained Frank Andrews, DVM, Dipl. ACVIM, LVMA Equine Committee Professor of Equine Medicine and director of the Equine Health Studies Program at Louisiana State University (LSU), in Baton Rouge. “For example, the veterinarian goes out to vaccinate a horse, and it comes down with the disease or becomes ataxic (incoordinated). Then they test the horse and it is positive. So, did the vaccine cause the positive titer or was it true disease?”

To get a better understanding, Andrews and colleagues at LSU and Boehringer-Ingelheim Vetmedica (BI), looked at a killed-antigen vaccine (BI’s Vetera Gold) designed to protect against EEE/WNV. Both viruses are endemic in the United States and can cause ataxia and acute death, especially in the case of EEE, with infections occurring only seven to 14 days or four to 10 days, respectively, after exposure to the virus.

Andrews was interested in two antibodies produced by specialized white blood cells called lymphocytes, immunoglobulins IgG and IgM